In general, wheel bearing apparatii is classified into a so-called first, second, third and fourth generation type. In a first generation type, the wheel bearing includes double row angular-contact ball bearings fit between the knuckle and the wheel hub. In a second generation type, a body mounting flange or a wheel mounting flange is integrally formed on the outer circumference of an outer member. In a third generation type, one of the inner raceway surfaces is directly formed on the outer circumference of the wheel hub. In a fourth generation type, the inner raceway surfaces are directly formed on the outer circumferences, respectively, of the wheel hub and the outer joint member of a constant velocity universal joint.
Recently, there has been a strong desire to improve fuel consumption in view of resource savings or anti-pollution. Reduction of weight in automobile parts, particularly in a wheel bearing apparatus, has been noticed. In response of these demands, there is a strong desire to reduce the weight of the wheel bearing apparatus while maintaining its strength and rigidity. A wheel bearing apparatus 50 has been proposed, as shown in FIG. 5, that can solve these objectives.
The wheel bearing apparatus 50 is a third generation type used for a driven wheel. It has a wheel hub 51, an inner ring 52, an outer member 53, and double row balls 54, 54. The wheel hub 51 has, on its one end, an integrated wheel mounting flange 55. An inner raceway surface 51a is formed on the outer circumference of the wheel hub 51. A cylindrical portion 51b axially extends from the inner raceway surface 51a. Furthermore, hub bolts 55a are secured equidistantly along the periphery of the wheel mounting flange 55.
The inner ring 52 is formed with an inner raceway surface 52a on its outer circumference. The inner ring 52 is press-fit onto the cylindrical portion 51b of the wheel hub 51. Axial falling out of the inner ring 52, from the wheel hub 51, is prevented by plastically deforming an end of the cylindrical portion 51b of the wheel hub 51. This forms a caulked portion 51c. 
The outer member 53 is formed with a body mounting flange 53b on its outer circumference. The body mounting flange 53b is mounted on a knuckle (not shown). The inner circumference of the outer member includes double row outer raceway surfaces 53a, 53a. The double row balls 54, 54 are rollably contained between the double row outer raceway surfaces 53a, 53a and the double row inner raceway surfaces 51a, 52a. 
As shown in FIG. 6, the body mounting flange 53b, of the outer member 53, is formed with a plurality of bolt insertion bores 56, 57. Bolts to be fastened to a body of a vehicle are inserted through the bores 56, 57. The total number of the bolt insertion bores 56, 57 is four. Two bores 56 are arranged at an upper portion, at a position below the uppermost position of the body mounting flange 53b, at an angle of about 60° of the body mounting flange 53b. The other two bores 57 are arranged at a lower portion positioned above the lowermost position of the body mounting flange 53b by an angle of about 45°.
The thickness of upper and lower portions 58, that are positioned between two upper bolt insertion bores 56, 56 and between two lower bolt insertion bores 57, 57, is the same as a thickness of the peripheral portion 59 around the bolt insertion bores 56, 57. The thickness of the upper and lower portions 58 is made thicker than the fore and aft portions 60.
When the total number of the bolt insertion bore is three, two of them are arranged on both the upper sides of the uppermost portion of the body mounting flange 53b. One of them is arranged at a position near the lowermost portion of the body mounting flange 53b. The thickness of the peripheral portion around the bolt insertion bores is not thinned. The thickness of the peripheral portion around the lower bolt insertion bore is thinner than those of the peripheral portions around the two upper bolt insertion bores.
The wheel bearing apparatus 50 receives vertical loads, lateral loads and fore and aft loads. The largest moment is applied to the upper and lower portions 58 of the body mounting flange 53b when the lateral load is applied to the wheel bearing apparatus 50. Accordingly, the upper and lower portions 58 of the body mounting flange 53b are formed thicker. This improves the strength and rigidity of the body mounting flange 53b. On the other hand, the moment applied to the fore and aft portions 60 is relatively small. Thus, the fore and aft portions 60 do not require the same strength and rigidity as those of the upper and lower portions 58. Thus, it is possible to form the fore and aft portions 60 relatively thinner than the upper and lower portions 58 to reduce the weight of the body mounting flange 53b. This balances the strength, rigidity and light weight at a higher level. See JP 2007-71352 A.
However, in the prior art wheel bearing apparatus 50, the bolt insertion bores 56, 57 are simple threadless insertion bores and not tapped bores. Thus, the outer member 53 can be secured to the knuckle by inserting fastening bolts (not shown) through the bolt insertion bores 56, 57 from the outboard-side, the wheel mounting flange 55 side. The fastening bolts are secured to tapped bores in the knuckle. In this case, a bolt seating surface 62 is required on the outboard-side surface 61 of the body mounting flange 53b, as shown in FIG. 7, in order to improve the mounting accuracy.
As shown in FIG. 8, the outer member bolt seating surface 62 is formed by lathe turning, shown by a dot-and-dash line, after hot forging, shown by two-dot chain line, and bored by using a boring jig such as a drill. A chamfered portion 63 is formed on an inboard-side surface 64 of the body mounting flange 53b. A pilot portion 65 and outer raceway surfaces 53s etc. are formed by lathe turning.
As described above, the bolt seating surface 62 is formed by lathe turning. However, a re-chucking operation of the outer member 53 is required to carry out the lathe turning of the bolt seating surface 62. Thus, problems occur such as increasing the number of processing steps and cycle time and therefore the manufacturing cost.